(a) Field of the Invention
The present invention relates to a distributed feedback (DFB) semiconductor laser device having a longer wavelength band between 0.9 and 1.65 xcexcm frequently used as a light source for optical communication, more in detail to the DFB semiconductor laser device having a stable emission wavelength and being operatable at a single wavelength especially suitable for exciting an optical fiber amplifier or a Raman amplifier, and to the Peltier-fee DFB semiconductor laser device having smaller temperature dependency.
(b) Description of the Related Art
A DFB semiconductor laser device has a stable laser emission wavelength and is operated at a single wavelength especially suitable for use in wave-lenght-multiplexing (WDM) systems and high-bit-rate transmission and so on.
As the first conventional example, the configuration of a 980 nm-band InGaAs-based semiconductor laser device used as an excitation light source for an optical fiber amplifier will be described.
The semiconductor laser device which is a conventional Fabry-Perot semiconductor laser device includes a stacked structure having an n-AlGaAs cladding layer, an InGaAs/GaAs quantum well layer, a p-AlGaAs cladding layer having a thickness of 2 xcexcm and a p-GaAs capping layer sequentially stacked on an n-GaAs substrate having a thickness of 100 xcexcm.
The top portion of the p-AlGaAs cladding layer and the p-GaAs capping layer in the stacked structure include mesa stripes having a thickness of 4 xcexcm.
An SiN passivation films is formed on the p-AlGaAs cladding layer and on the sidewalls of the mesa structure excluding the top surface of the p-GaAs capping layer.
A Ti/Pt/Au stacked metal film acting as a p-electrode is deposited on the top surface of the p-GaAs capping layer and the SiN passivation film, and an AuGe/Ni/Au stacked metal film acting as an n-electrode is deposited on the bottom surface of the substrate.
A cavity length is 100 xcexcm. An anti-reflection(AR) film having a reflection rate of 7% is formed on the front cleavage facet, and a higher reflection (HR) film having a reflection rate of 95% is formed on the rear cleavage facet.
The excitation wavelength of an Er (erbium) doped fiber amplifier attracting attention in these days is generally 980 nm, and the window of the excitation wavelength is extremely low in view of the excitation efficiency.
The 980 nm semiconductor laser used as the excitation optical source is desirably stable in wide ranges of temperature and injected current and operates at an emission wavelength of 980 nm. Further, in view of the excitation efficiency, the emission spectrum characteristic is desirably a single mode rather than a multiple mode.
Accordingly, a refractive index-coupled DFB laser having the single mode spectrum characteristic and stably operating in spite of the change of an operating circumstance is more important than the Fabry-Perot type laser operating with a multi-mode.
A refractive index-coupled DFB laser of a second conventional example includes a stacked structure having an n-AlGaAs cladding layer, an InGaAs/GaAs quantum well layer, a GaAs waveguide layer, a p-AlGaAs cladding layer having a thickness of 2 xcexcm and a p-GaAs capping layer sequentially stacked by using an MOCVD method on an n-GaAs substrate.
A diffraction grating having a cycle of 140 nm is formed on the GaAs waveguide layer by using electronbeam lithography.
The top portion of the p-AlGaAs cladding layer and the p-GaAs capping layer in the stacked structure include mesa stripes having a thickness of 4 xcexcm.
Similarly to the previous semiconductor laser device, an SiN passivation film is formed on the p-AlGaAs cladding layer and on the sidewalls of the mesa structure excluding the top surface of the p-GaAs capping layer, and a p-electrode and an n-electrode are formed.
A cavity length is 1000 xcexcm. An anti-reflection(AR) film having a reflection rate of 7% is formed on the front cleavage facet, and a higher reflection (HR) film having a reflection rate of 95% formed is on the rear cleavage facet.
The spectrum characteristics of the DFB laser were examined. The optical strength ratio called a side mode suppression ratio (SMSR) between a main emission mode xe2x80x9ca1xe2x80x9d and a side mode xe2x80x9ca2xe2x80x9d was relatively excellent, that is, 30 dB when injection current was 200 mA.
However, in the spectrum characteristics at the injection current of 400 mA, the two-mode emission (or mode skipping) was observed and the SMSR was reduced below 10 dB.
The spectrum stability of the DFB laser of the second conventional example was not high.
A GaInAsP-based semiconductor laser having a wavelength of a 1.3 xcexcm band or a 1.55 xcexcm band formed on an InP substrate has been generally used as an optical source in the field of optical communication.
The GaInAsP-based semiconductor laser includes a problem that a temperature characteristic of a threshold value or a characteristic temperature xe2x80x9cToxe2x80x9d which is a factor showing the temperature dependency of the threshold value is as low as 50 to 70 K. In other words, the temperature dependency is worse and a cooling device such as a Peltier device is required.
Further, in the DFB laser having a wavelength of a 1.3 xcexcm band or a 1.55 xcexcm band used for an optical source for CATV and so on in addition to a module for domestic transmission, the miniaturization and the lower power consumption are demanded and a semiconductor laser with excellent temperature characteristic is required operating at a higher wavelength band without a cooling device such as a Peltier free device.
As described in the second conventional example, the range of realizing the single mode is narrow and the DFB laser is unstable to external reflection rays. The yield of the DFB having the excellent spectrum characteristics is low.
Although an absorption type DFB laser having a higher yield has been demanded, a GaInAs layer having a thickness of about 50 nm and an In content of about 30% is necessary as an absorption layer for absorbing an emission wavelength of a GaInAs/GaAs active layer for realizing the absorption type DFB laser.
However, due to the lattice-mismatching, the GaInAs layer having the In content of about 30% and thickness of about 50 mm can be hardly grown with the higher quality crystal growth.
In one aspect of the present invention, a DFB semiconductor laser device is provided which includes: a semiconductor substrate; and an active layer and a diffraction grating overlying the semiconductor substrate, the diffraction grating having a composition of GaInNAs(Sb) and absorbing light having a laser emission wavelength of the active layer (hereinafter referred to as xe2x80x9cfirst inventionxe2x80x9d).
In accordance with the first invention, the DFB semiconductor laser device having a higher side mode suppression ratio can be provided which stably operates in a wider range of injection current when external reflection light is incident by proving the diffraction grating formed by the GaInNAs(Sb) having the composition for efficiently absorbing light which has the laser emission wavelength of the active layer.
In another aspect of the present invention, a DFB semiconductor laser device is provided which includes: a GaAs substrate; a quantum well structure including a GaInNAs(Sb) quantum well layer acting as an active layer; and an absorption layer including a diffraction grating made of GaInNAs(Sb) having a band-gap energy which is lower than that of the GaInNAs(Sb) quantum well layer (hereinafter referred to as xe2x80x9csecond inventionxe2x80x9d).
In a further aspect of the present invention, a DFB semiconductor laser device is provided which includes: a a GaAs substrate; a quantum well structure including a GaInNAs(Sb) quantum well layer acting as an active layer; and an absorption layer including a diffraction grating made of GaInNAs(Sb) having a band-gap energy which is larger than that of the GaInNAs(Sb) quantum well layer by 40 meV or less (hereinafter referred to as xe2x80x9cthird inventionxe2x80x9d).
In accordance with the second and the third inventions, the gain coupled-type DFB semiconductor laser device having the absorption wavelength band of 0.9 to 1.65 xcexcm can be provided which operates at a higher single mode and has a higher temperature performance, a higher product yield and no Peltier effect. The DFB laser device can be realized by forming the absorption layer of the diffraction grating by the GaInNAs having the band-gap energy lower than that of the GaInNAs quantum well layer or by the GaInNAs translucent with respect to the laser emission wavelength of the active layer having the band-gap energy somewhat larger than that of the GaInNAs quantum well layer.
In these inventions, a similar effect can be obtained when the GaInNAsSb is used in place of the GaInNAs.
The above and other objects, features and advantages of the present invention will be more apparent from the following description.